使用jpeg库读取jpg文件

今天使用jpeg库读取jpg文件, 编译过程中遇到了不少错误.

开始时运行到 jpeg_create_decompress(&cinfo) 函数执行不成功. 这个问题使用其他的 jpeg.lib解决了, 但接下来是

(void) jpeg_read_header(&cinfo, TRUE); 函数访问内存错误.这个问题一直解决不了.后来直接在别人一个已经使用jpeg库读jpg文件的程序上进行修改, 读取部分的代码还是一样,但却能运行了. 这应该是程序设置的问题. 但对比两个程序的设置,现在还是找不到区别,这留以后研究. 把代码贴出来如下:

struct my_error_mgr {
struct jpeg_error_mgr pub; /* "public" fields */

jmp_buf setjmp_buffer; /* for return to caller */
};

typedef struct my_error_mgr * my_error_ptr;

METHODDEF(void)
my_error_exit (j_common_ptr cinfo)
{
/* cinfo->err really points to a my_error_mgr struct, so coerce pointer */
my_error_ptr myerr = (my_error_ptr) cinfo->err;

/* Always display the message. */
/* We could postpone this until after returning, if we chose. */
(*cinfo->err->output_message) (cinfo);

/* Return control to the setjmp point */
longjmp(myerr->setjmp_buffer, 1);
}

bool Image::load(const char *filename)
{
std::string file(filename);
if (file.substr(file.length() - 3, file.length()) != "jpg")
return 0;

struct jpeg_decompress_struct cinfo;
struct my_error_mgr jerr;
FILE * infile;
JSAMPARRAY buffer;
int row_stride;

if ((infile = fopen(file.c_str(), "rb")) == NULL) {
fprintf(stderr, "can't open %s/n", file.c_str());
return 0;
}

cinfo.err = jpeg_std_error(&jerr.pub);
jerr.pub.error_exit = my_error_exit;

if (setjmp(jerr.setjmp_buffer)) {
jpeg_destroy_decompress(&cinfo);
fclose(infile);
return 0;
}

jpeg_create_decompress(&cinfo);
jpeg_stdio_src(&cinfo, infile);
(void) jpeg_read_header(&cinfo, TRUE);
(void) jpeg_start_decompress(&cinfo);

row_stride = cinfo.output_width * cinfo.output_components;
const int component = cinfo.output_components;

buffer = (*cinfo.mem->alloc_sarray)
((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1);

w = cinfo.output_width;
h = cinfo.output_height;

create(w,h);
int y=0;
while (cinfo.output_scanline < cinfo.output_height) {
(void) jpeg_read_scanlines(&cinfo, buffer, 1);

if (component==3)
{
for (int i = 0; i < w; ++i)
set_pixel(i,h-y-1,(unsigned char)buffer[3*i+0],(unsigned char)buffer[3*i+1],(unsigned char)buffer[3*i+2]);
}else if (component==1)
{
for (int i = 0; i < w; ++i)
set_pixel(i,h-y-1,(unsigned char)buffer[3*i],(unsigned char)buffer[3*i],(unsigned char)buffer[3*i]);
}

y++;
}
(void) jpeg_finish_decompress(&cinfo);
jpeg_destroy_decompress(&cinfo);
fclose(infile);

return 1;
}